The present invention relates to a borehole data transmission apparatus and more particularly to a borehole data transmission apparatus utilizing a centrifugal pump impeller to provide a turbine stage having substantial pressure response characteristics for the pressure pulsing of data from the borehole through the drilling mud to the surface of the earth.
It is well known to transmit measurement data from a measuring instrument at the downhole end of a drill string within a borehole by generating pressure variations within the mud flow passing along the drill string and to retrieve the transmitted data by sensing such pressure variations at the surface. Typically such data includes weight on the bit, RPM, natural gamma, formation resistivity, bottom hole temperature, bottom hole pressure, and virtually any information related to detectable conditions. Such information is usually transmitted from the bottom of the borehole to the surface as a series of hydraulic pulses produced in, and transmitted through, the drilling fluid. The information, in the form of pulses, may be transmitted to the surface without disturbing the normal drilling operations or ceasing the flow of drilling fluid. At the surface, the pressure pulses are detected, usually converted to electrical signals, and processed to provide the sought for information in a useable format.
It is common to create pressure pulses in drilling fluid by periodically interrupting the normal flow of the fluid through the drill string, or diverting a portion of the flow into the annulus of the drill string, to form a series of pulses in the drilling fluid which is normally pressurized and which normally flows down through the drill string and back up the annulus around the outer surface of the drill string.
Thus, for example, a variety of systems have been used to form the pressure pulses and to effect transmission to the surface. In most instances, the pulsing system involves some form of valve so positioned that the entire fluid flow through the drill string also flows through the valve. The recognized difficulty with this arrangement is that the drilling fluid, due to its composition, the rate of flow and volume thereof, tends to be quite abrasive. As a result of the abrasive conditions of the drilling fluid and the fact that the transmission of data may involve a significant number of pulses, the life of the valves is somewhat limited.
A number of U.S. Patents show the use of rotary "turbine-like" valves which include a rotor and a stator. Both the rotor and stator have slots which can be aligned to open the valve and let drilling fluid pass through or misaligned to close the valve and provide a strong resistance to the passing of the drilling fluid. U.S. Pat, No. 4,956,823 of Russell shows a signal transmitter comprising main and secondary impellers angularly displaceable relative to one another in response to a change in load of a generator so as to vary the flow passage area and therefore the pressure drop across the rotating impeller assembly. U.S. Pat. No. 3,705,603 of Hawk refers to "a motor actuated rotary valve" which turns between an open position and a closed position in a rotary fashion. Turbines are also used for generating the electrical power needed to operate the logging-while-drilling apparatus. This is shown in the patents referenced, but is also used with non-rotary valves to create the pres-sure pulses as shown in U.S. Pat. No. 3,737,843 of Le Peuvedic et al. Numerous other patents show the use of turbines for generating power in a downhole logging-while-drilling apparatus and also show the use of rotary turbine-like valves which operated in an open and shut mode for generating the pressure pulses. Those systems shown in the patents referenced which use rotary valves for periodically interrupting a drilling fluid in order to generate pulses are motor actuated with the electrical power generated by a separate turbine motor.
The Applicant of the present invention is also the inventor of a logging-while-drilling tool shown in U.S. Pat. No. 4,734,892 which includes a variable rotation device in the form of a turbine rotor(s) of an axial turbine stage(s) with a changing pressure drop across the stage depending on the rotor rotational speed (RPM). The variability of the turbine rotor RPM is caused by changing of torsional loading torque on the rotor shaft. Such variable rotation of the turbine rotor generates the variable pressure drop across the turbine stage(s) to form the pressure wave signal in the drilling fluid.
The degree of torsional loading required on the rotor shaft in order to generate the pressure wave signal, is a function of the pressure response characteristics of the rotational device. Rotational devices are known which have different pressure response characteristics with respect to rotor rotational speed. A rotational device without significant pressure response characteristic will require significant torsional torque to generate the variable pressure drop across the turbine stage(s) thereby resulting in reducing the useful life of the device due to the friction and stress in the rotor shaft. Alternatively, multiple rotational elements could be configured to provide multiple turbine stages, the cumulative effect thereby creating pressure response characteristics across the several turbine stages. However, the downhole data transmission devices having multiple turbine stages must be of a relatively large axial size in order to generate the required significant pressure response across the several turbine stages for the data transmission.
It is an object of this invention to provide a simple, inexpensive, and compact borehole data transmission device having substantial pressure response characteristics for borehole pulse data transmission.
It is another object of the present invention to provide a borehole data transmission device having substantial pressure response characteristics across the turbine stage(s) thereby minimizing the number of turbine stage(s) required, resulting in a compact data transmission device.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.